TN (twisted nematic) liquid crystal cells having a twist angle of 90.degree. have heretofore been used as display terminals of about 1/100 duty cycle, such as watches and electric calculators. In view of the principle, however, it has been believed that it is difficult to increase the number of duty cycles to more than the above value, because it is accompanied by a reduction of display quality.
On the contrary, a supertwisted birefringence effect mode utilizing the birefringence of a liquid crystal cell in which the highest possible twist angle to produce bistability, one of electro-optic characteristics of a chiral nematic liquid crystal, is controlled between 180.degree. and 270.degree. can be used as a liquid crystal display device of about 1/400 duty cycle. This system is hereinafter referred to as an "SBE mode", including supertwisted nematic modes using an aligning organic film, called an STN mode and an HBE mode.
T. J. Scheffer and J. Nehling disclosed in Appl. Phys. Lett., 45, 1021 (1984) that the above SBE mode can provide a matrix display in which a display device is driven in the same driving manner as that of a matrix of the conventional 90.degree. twisted TN mode, i.e., the fast scan addressing mode responding to the effective value of applied voltage. It is also reported that the SBE mode display has a much higher contrast and a much wider viewing angle than the 90.degree. twisted TN display.
Liquid crystal compositions to be used in liquid crystal displays of the SBE mode are required to have the following characteristics:
(1) a steep voltage-transmittance curve,
(2) a high nematic-isotropic liquid phase transition temperature, i.e., clearing point (NI), and
(3) a low viscosity (.eta.).
The characteristic (1) above is necessary for increasing the display contrast of liquid crystal displays of the SBE mode
The characteristic (2) is needed to avoid coloring of the displays due to temperature dependency of refractive index anisotropy in the SBE mode; it is desirable that the clearing point be as high as possible.
In liquid crystal cells of the SBE mode, it is desirable that the viscosity be as low as possible, because the response time is in proportion to .eta..d.sup.2 wherein d indicates a cell thickness.
In fact, however, the conventional liquid crystal compositions fail to satisfy the above requirements; a liquid crystal composition satisfying the above requirements and thus suitable for use in displays of the SBE mode has not been found heretofore.